4-Methoxybenzamidinium bromide

The title salt, C8H11N2O+·Br−, was synthesized by the reaction between 4-methoxybenzamidine (4-amidinoanisole) and hydrobromic acid. In the cation, the amidinium group has two similar C—N bonds [1.304 (2) and 1.316 (2) Å], and its plane forms a dihedral angle of 31.08 (5)° with the benzene ring. The ions are associated in the crystal into a three-dimension hydrogen-bonded supramolecular network featuring N—H+⋯Br− interactions.


4-Methoxybenzamidinium bromide Simona Irrera and Gustavo Portalone Comment
As part of our ongoing interest in systematic structural analysis of proton-transfer adducts containing molecules of biological interest (Portalone, 2011;Portalone & Irrera, 2011) this study reports the single-crystal structure of the title molecular salt, 4-methoxybenzamidinium bromide, (I), which was obtained by a reaction between 4-methoxybenzamidine (4-amidinoanisole) and hydrobromic acid in water solution. Benzamidine derivatives, which have shown strong biological and pharmacological activity (Powers & Harper, 1999), are being used in our group as bricks for supramolecular construction (Portalone, 2010;Portalone, 2012). Indeed, these molecules are strong Lewis base and their cations can be easily anchored onto numerous inorganic and organic anions and polyanions, largely because of the presence of four potential donor sites for hydrogen-bonding.
In the cation the amidinium group forms a dihedral angle of 31.08 (5)° with the benzene ring, which is close to the values observed in protonated benzamidinium ions (23.2-30.4°; Portalone, 2010;Portalone, 2012). The lack of planarity in all these systems is obviously caused by steric hindrances between the H atoms of the aromatic ring and the amidine moiety. This conformation is rather common in benzamidinium-containing small molecule crystal structures, with the only exception of benzamidinium diliturate, where the benzamidinium cation is planar (Portalone, 2010). The pattern of bond lengths and bond angles of the 4-methoxybenzamidinium cation agrees with that reported in previous structural investigations (Portalone, 2010;Portalone, 2012;Irrera et al., 2012;Irrera & Portalone, 2012a, 2012b, 2012c, 2012d. In particular the amidinium group, true to one's expectations, features similar C-N bonds [1.304 (2) and 1.316 (2) Å], evidencing the delocalization of the π electrons and partial double-bond character.
Analysis of the crystal packing of (I), (Fig. 2), shows that each amidinium unit is bound to three bromide anions by four distinct weak N-H + ···Brhydrogen bonds (N + ···Br -= 3.3163 (19)-3.4765 (17) Å; Table 1). The ion pairs of the asymmetric unit are joined by two N-H + ···Brhydrogen bonds in ionic dimers, where Branion acts as a bifurcated acceptor, thus generating an R 1 2 (6) motif (Bernstein et al., 1995). These subunits are then joined through the remaining N -H + ···Brhydrogen bonds to adjacent Branions leading to the formation of three-dimension hydrogen-bonded network.
Experimental 4-Methoxybenzamidine (1 mmol, Fluka at 96% purity) was dissolved without further purification in 6 ml of hot water and heated under reflux for 6 h. While stirring, HBr (2 mol L -1 ) was added dropwise until pH reached 2. After cooling the solution to an ambient temperature, colourless crystals suitable for single-crystal X-ray diffraction were grown by slow evaporation of the solvent after four weeks.

Refinement
All H atoms were identified in a difference Fourier map, but for refinement all C-bound H atoms were placed in calculated positions, with C-H = 0.93 Å (phenyl) and 0.96 Å (methyl), and refined as riding on their carrier atoms. The U iso values were kept equal to 1.2U eq (C) or 1.5U eq (C) for methyl H atoms. The hydrogen atoms of the methyl group were allowed to rotate with a fixed angle around the C-C bond to best fit the experimental electron density [HFIX 137 in the    Special details Geometry. All s.u.'s (except the s.u. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell s.u.'s are taken into account individually in the estimation of s.u.'s in distances, angles and torsion angles; correlations between s.u.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell s.u.'s is used for estimating s.u.'s involving l.s. planes. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > 2σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq